Color correction is conducted prior to printing performed by a printing press. The color correction is conducted in order to check finish of printing, for example, 1) the presence or absence of a typographical error or gap, 2) the presence or absence of dust, a flaw, a smear or the like, 3) gradation reproducibility such as brightness or darkness of printed matter, 4) the color reproducibility of the printed matter, 5) the sharpness of the printed matter, and the like.
The color correction is generally conducted on the basis of proof printing performed by a proof press. In particular, computerization of pre-printing processing has been progressed in recent years, and therefore, there has been prevailed a digital proof press which can produce proof print directly based on digital data by means of an ink jet system or a sublimation thermal ink-transfer system.
In this case, color matching method is an important technique, in which a color reproducing region of the proof print produced by the proof press is approximated to that of printed matter to be produced by a printing press, thereby producing a visually similar result.
However, in many cases, the respective color reproducing regions inherent to the printed matter and the proof print are different from each other, and thus, it is very difficult to accurately approximate the color reproducing regions to each other.
For example, black (Bk) ink is generally used in addition to three primary colors in a subtractive process, i.e., cyan (C), magenta (M) and yellow (Y) in a printing process. The use of the Bk ink produces an economic effect such as ink saving by expressing an Indian ink character mainly used on a printing paper with monochromatic color ink as well as a quality improvement effect owing to an increase in density region in a high density part or color reproducing region.
There are two methods for making the black Bk from the three colors C, M and Y: a full-black method and a skeleton black method.
Assuming that the colors C, M and Y are uniformly mixed into the black Bk, a value min(CMY), i.e., a minimum value out of values of the colors C, M and Y is replaced with the black Bk, thereby achieving representation with the three colors. This is a method for maximizing a value replaced with the black Bk, and is referred to as the full black method. In an example illustrated in FIG. 5, out of the values of the three colors, i.e., C being 72%, M being 94% and Y being 43%, the minimum value Y is replaced with the black Bk, and further, is subtracted from the values of C and M. In this case, the amount equal to 43%×2 of ink is saved.
In the full-black method, scum of the black Bk ink occurs at a highlight part, which, therefore, looks dirt. Thus, as illustrated in FIG. 6, the value min(CMY) is not wholly replaced with the black Bk, but partly remains. This method is referred to as the skeleton black method.
The ink colors C, M, Y and Bk are relative to each other. The ink colors C, M, Y and Bk of printed matter produced by a printing press are different from those of proof print produced by a color printing press. Moreover, even the ink colors C, M, Y and Bk of one and the same printed matter slightly look different according to manufacturers. In view of this, the colors cannot be simply compared with each other as they are. Additionally, in comparison of the colors between a monitor and a color printing press, three colors R, G and B are used in the monitor, and therefore, respective color spaces in the monitor and the color printing press are different from each other.
Therefore, there has been conceived a method for, on the presumption of an absolute color space independent of color representing means, replacing color represented objects in the absolute color space so as to compare them with each other. This is referred to as a device independent color.
The absolute color space originally signifies an xyz space which is obtained by numerating a stimulus value with respect to human eyes. Since this space is a markedly distorted space, it is not suitable for perceiving a color difference. Therefore, there has been proposed a space rearranged in a uniformly three-dimensional space (a uniform color space). Currently, an Lab space is mainly used as the uniform color space. Since the uniform color space is the Euclidean space, the color difference is readily obtained.
In actual color management, all of the color of the printed matter by the printing press, and the color of the proof print by the color printing press and the color of the monitor are measured by a spectrophotometer, and then, their color reproducing regions are represented with the Lab values. Here, the colors can be simply compared with each other because of the same color space. Consequently, one of the color reproducing regions is compressed to the other, thereby achieving color matching. This is referred to as Gamut Mapping.
Here, since the color space after the color matching is the Lab space, the Lab space is converted again into respective relative color spaces, thereby obtaining actual conversion values. FIG. 7 illustrates an example in which the above-described procedures are applied to the color matching between the printed matter and the proof print.
In this manner, the device independent color is a remarkably effective color management technique. However, there arises a serious problem in view of representation of the black color. That is, since the black is a color which is artificially created, the original value of the black Bk is lost when the colors C, M, Y and Bk are converted to the colors in the Lab space, which are further converted again to the colors C, M, Y and Bk. Namely, in this case, there exists no reversibility between the colors C, M, Y and Bk before and after the conversion.
The most adverse effect on the actual processing is that a part having an area factor of 100% of the black mainly used in the representation of an Indian ink character or the like (C=M=Y=0, Bk=100) is lost, resulting in three or four colors (for example, C=71, M=75, Y=85 and Bk=35). In other words, the Indian ink character cannot be sharply represented, and further, the ink need be used in a greater quantity.
Moreover, in the printing process, in the case where color proof of offset lithography or gravure printing is performed by the use of a color printing press, a paper which can be used in the color printing press for performing the proofing is different from a paper which can be used in a printing press for performing main printing (hereinafter referred to as “a printing paper”) in most cases. In particular, special papers need be used in an ink jet printing press, a sublimation thermal ink-transfer type printing press or the like, and therefore, the same paper as the printing paper cannot be used in fact.
If printing papers are different, it is more difficult to approximate the proof print to the printed matter.
In view of this, there has been conventionally a method for reproducing the color of the printing paper on image data so as to visually approximate the proof print to the printing paper. In this method, it is possible to reproduce the printing paper in proofing with relative ease unless a paper special to the color printing press has so extreme a color as to prevent reproducibility of the color of the printing paper, thereby obtaining the proof print which can be more approximated to the printed matter.
However, in this method, there arises a problem not in a region on the proof print, to which no ink is transferred, i.e., a blank region, but in a region to which the ink is transferred, as follows: if the color of the paper is added to only the blank region to which no ink is transferred, the color of the paper is not added to the region to which the ink is transferred, and therefore, the original color remains as it is.
Since printing gradation representation is principally area gradation, the color of the ink and the color of the paper basically looks mixed in a visual way. In other words, the color of the paper visually has an effect on the color of the ink. This effect is particularly shown more at a highlight part having a low ink area factor, so that the effect of the color of the paper visually becomes greater.
Another method for approximating the proof print to the printed matter in the case where the printing papers are different is a method for uniformly adding the color of the paper to the entire printing image. This method is further classified into a method for adding the color of the printing paper to image data and a method for printing a printing image after previous printing of the color of the printing paper over the entire special paper. In the above-described methods, the color matching between the proof print and the printed matter becomes excellent at the highlight part with relative accuracy, but the exposure area of the color of the paper is small from an intermediate part to a shadow part, and the color of the paper largely has an effect even on a region which hardly gets an effect by the color of the paper, whereby a color reproducing region of the proof print is markedly deviated from a color reproducing region of the printed matter.
Thus, a problem to be solved by the present invention is to improve the color reproducibility of the printed matter by means of the printing press in proofing by the proof press, in particular, by the color printing press.
Moreover, another problem to be solved by the present invention is to reproduce the color of the printing paper by means of the printing press, so as to reproduce the representing characteristics of the black ink on the printed matter.